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Rotary Tables

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					                                  Turning the Tables
Rotary tables are used to impart circular motion to a workpiece lying flat on a milling
machine table. Much work that previously was done using the rotary table, is now done in
the X-Y plane (G17) of CNC machines. It is still necessary at times to use a rotary table in
manual machining, for example tool making and mold making. In addition one off pieces
can be made efficiently with a rotary table and that prompts the explanation that follows.

The group of tools known as rotary tables or circular milling attachments divides the circle
in angular terms—that is by degrees and fractions of degrees. The rotary table is an
extremely useful accessory for the milling machine. A rotary table consists of a precision
worm and wheel unit that can be attached to a milling machine table. Rotary tables may
be designed so they can be mounted on a machine tool either vertical or a horizontal
position.

Parts
         Work table: The work table is a circular working area with t-slots with which clamps
          are used to mount a work piece.
         Crank: The crank handle rotates the table clockwise and counterclockwise. The
          handle on this table is geared so that one revolution of the crank handle turns the
          rotary table five degrees. There are micrometer divisions of five degrees around the
          handle. The table also has markings of 360 degrees
         Clamp: There are two clamp levers to keep the table fixed once an angle has been
          found.
         Base. The rotary table has two bases. One for a vertical axis of rotation as shown in
          the video and one for horizontal axis of rotation as shown in the above image.
         Indexing pin: The indexing pin is a fast way to find and lock onto common angles.
          This rotary table has an indexing detent every five degrees of table rotation. A close
          look below shows the indexing pin not engaged and then engaged. The indexing pin
          will likely be spring loaded with a twist lock to keep it out of the indexing slots.




WEC 04/06 MET 355
Positioning a part or feature of a part precisely on a rotary table requires two separate and
sequential steps.
First and most important is to position the part over the exact centerline of the true table
axis. This is done by spinning the rotary table and carefully tapping the work piece as the
dial indicator readings are observed. When zero deflection of the needle is acquired (or as
close as possible given the part) the work is directly over the axis of the rotary table. It is
not necessary to mount the indicator in the machine spindle, and in some instances not
possible to accomplish this with spindle mounting, but it can be.

Most rotary tables have a feature that will allow the table to be turned freely by hand with
disengagement of the worm gears. Spinning the table by hand speeds up the part
positioning on center axis of the table. Use this method to advantage when possible, as
the time spent centering parts can become cumbersome.

Second is the positioning of the milling machine spindle directly over the axis of the
workpiece. This is accomplished by spinning a dial indicator in or around the workpiece
and adjusting the tables (X-Y axis) until the reading is zero or as close as is possible. This
task requires the indicator be mounted in the spindle.




WEC 04/06 MET 355
It is of course possible to indicate and position the machine spindle directly over the rotary
table center hole, then position the workpiece in line with the hole axis. It is possible to do
this but it is not advisable. Using this method is seldom if ever as precise as following the
recommendations first outlined. Rotary table hole centers are often not exactly on the true
axis of the table itself. This is more common with the lower cost import tables now
common in many plants. Consider the fact that two features must be indicated to locate
the part; the rotary table hole and the workpiece. Stacking measurements can accumulate
error, and often does.

To check the concentricity of a rotary table relative to its true axis, indicate the hole while
spinning the table. Most likely it will be off from a few tenths of a thousandth to a few
thousandths of an inch. The accuracy depends a great deal on the quality of the table.
When indicating on a rotary table or lathe the readings are TIR (total indicator reading).
For example, a reading of .004” would mean the hole is only .002” off the true center axis.
Precision is relevant to the task at hand. A few thousandths from true position is irrelevant
when building bicycle frames, critical for airplane parts.

Machine precision parts carefully
Holding close tolerances on diameters internally or externally on a rotary table is more
demanding. The same process that halves the error when indicating a part to the table
axis, also doubles the error when cutting chips. One thousandth movement of the milling
machine table axis will in theory remove two thousandths off the diameter of a part
clamped on a rotary table.

In addition to the “error doubling” concept, the amount of tooling bolted together for rotary
table work is inherently less rigid than most setups used on a milling machine. Use
caution when attempting precision work on a rotary table. It is possible to accomplish very
accurate work if the limitations of rigidity are compensated with procedures; sharp cutters,
slow feeds, light cuts and allowance for cutter flex.




WEC 04/06 MET 355

				
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